See Principles of Operation for a complete description of how these instructions function. The following paragraphs provide a general description of these branch instructions.
The BAL and BALR instructions are unconditional branch instructions (to the address in operand 2). BAL and BALR function differently depending on the addressing mode in which you are executing. The difference is in the linkage information passed in the link register when these instructions execute. In 31-bit addressing mode, the link register contains the AMODE indicator (bit 0) and the address of the next sequential instruction (bits 1-31); in 24-bit addressing mode, the link register contains the instruction length code, condition code, program mask, and the address of the next sequential instruction.
BAS and BASR perform the same function that BAL and BALR perform when BAL and BALR execute in 31-bit addressing mode.
The BSM instruction provides problem programs with a way to change the AMODE bit in the PSW. BSM is an unconditional branch instruction (to the address in operand 2) that saves the current AMODE in the high-order bit of the link register (operand 1), and sets the AMODE indicator in the PSW to agree with the AMODE of the address to which you are transferring control (that is, the high order bit of operand 2).
The BASSM instruction functions in a manner similar to the BSM instruction. In addition to saving the current AMODE in the link register, setting the PSW AMODE bit, and transferring control, BASSM also saves the address of the next sequential instruction in the link register thereby providing a return address.
BASSM and BSM are used for entry and return linkage in a manner similar to BALR and BR. The major difference from BALR and BR is that BASSM and BSM can save and change addressing mode.
The BAKR instruction is an unconditional branch to the address in operand 2. In addition to the branching action, it adds an entry to the linkage stack.
For more information on the linkage stack, see System-provided linkage stack.